1. Field of the Invention
This invention relates to a magneto-optical disc.
2. Prior Art
Optical discs are of great interest as high capacity information carrying media. Among the optical discs, magneto-optical discs of the magnetic field modulation system are expected to find use in data files or the like. A magneto-optical disc is recorded in a magnetic field modulation mode by directing a laser beam from an optical head to the recording layer of the disc in a DC manner for raising the temperature thereof and simultaneously applying a modulated magnetic field across the recording layer from a magnetic head opposed to the optical head. Therefore, the magnetic field modulation mode allows for overwrite recording.
Recently a magneto-optical disc of the magnetic field modulation mode (known as a mini-disc) which can record and reproduce signals at a low linear velocity equivalent to the linear velocity (1.2 to 1.4 m/s) of compact discs (CD) was put into commercial practice. This magneto-optical disc can share a drive unit with read-only optical discs of the specification complying with the CD. The potential applications of the drive unit for this magneto-optical disc include a portable type where it is required to reduce power consumption. However, the magneto-optical disc of the magnetic field modulation mode needs a recording magnetic head as well as an optical head, often leading to an increased power consumption. Therefore, there is a desire for a disc which is so sensitive to a magnetic field as to enable recording in a low magnetic field. The low magnetic field recording enables to reduce the power consumption, which in turn, enables a size reduction of a battery. There is also obtained an advantage in designing of the magnetic head in that the number of coil windings in the magnetic head can be reduced.
However, conventional magneto-optical discs, for example, 3.5-inch magneto-optical discs complying with the ISO standard generally require a recording magnetic field of 200 Oe or higher in order to provide a satisfactory C/N ratio. On the other hand, the above-mentioned mini-discs are desired to reduce the recording magnetic field to below 100 Oe, preferably below 80 Oe in order to minimize power consumption, and a C/N of at least 46 dB must be achieved at such a low magnetic field strength. It is further desired that at least 46 dB C/N be achieved at a recording magnetic field strength of 60 Oe in the future.
Further, the mini-discs must be more resistant against high temperature and high humidity because their outdoor use is an important consideration. Moreover, since the mini-discs are on the start of being utilized as data discs for computers too, a further improvement in safe data storage is required.
Japanese Patent Application Kokai (JP-A) No. 313835/1992 proposes "a method for preparing a magneto-optical recording medium by successively forming at least a rare earth element-transition metal alloy thin film and a dielectric film on a substrate wherein after the rare earth element-transition metal alloy thin film is formed, the substrate is maintained in an atmosphere in an electric discharge state, the atmosphere containing at least oxygen gas or a gas of a compound containing oxygen as one constituent element" and alleges that high C/N is accomplished in a low recording magnetic field. However, rare earth elements are highly oxidizable materials as is well known in the art and an experiment made by the inventors revealed that it was very difficult to control a gas of a compound containing oxygen as one constituent element (CO.sub.2, etc.) to an optimum amount and it was difficult to achieve the desired properties in a consistent manner. Further, oxygen which is contained in the magnetic layer of rare earth element-transition metal alloy has an increased possibility to adversely affect reliability.
Besides, Ichitani et al., "Recording field sensitivity of magneto-optical disks using very thin exchange-coupled films", J. Magn. Soc., Jpn., Vol. 17, Supplement No. S1 (1993), pp. 196-200 describes that a high magnetic field sensitivity suitable for the magnetic field modulation mode is achievable by using a laminate film of GdFeCo and TbFeCo as the recording film. Since this method increases the number of steps, cost reduction is difficult.